Alternating current (AC) type electric railroad vehicles (hereinafter, referred to as electric vehicles) obtain AC power from AC overhead wiring through a power collector such as a pantograph, and steps down the AC power with a main transformer, thereby obtaining an AC power source for an AC power conversion device. The AC power conversion device includes a converter that converts AC power into direct current (DC) power, an inverter that converts the DC power into AC power to drive a main electric motor, and a DC intermediate capacitor that connects the converter with the inverter. For the control of the converter, a pulse width modulation (PWM) scheme is applied, and a highly efficient power-factor-of-one control that makes the power factor of an input power to the converter to be as close to 1 as possible is performed.
In order to realize the highly efficient power-factor-of-one control, a microcomputer controller in the converter detects overhead wire voltage from a primary circuit of the main transformer, and performs a control in such a way that the input voltage of the converter and the overhead wire voltage have the same phase. In order to precisely detect the phase of the overhead wire voltage, it is necessary to detect the frequency of the overhead wire voltage.
At the time of a blackout, a PWM converter operates as a regenerative inverter, that is, as a power generator, and operates in synchronization with the output frequency thereof. Such operation causes the deviation of the output frequency from a frequency range of the overhead wire voltage, and conventionally the deviation is detected. In order to enable a changing in threshold for detecting the deviation of the output frequency for each territory where the frequency of the overhead wire voltage differs, it is necessary to detect the frequency of the overhead wire voltage.
Hence, Patent Literature 1 discloses a frequency detector for a train that is provided with the same number of band-pass filters (BPFs) as the types of overhead wire power source frequencies. This frequency detector has the center frequencies of the BPFs set to be consistent with respective frequencies of the power sources, and detects the center frequency of the BPF with the largest output as the overhead wire frequency.